Accelerating the Phosphatase-like Activity of Uio-66-NH2 by Catalytically Inactive Metal Ions and Its Application for Improved Fluorescence Detection of Cardiac Troponin I

Compared with natural enzymes, nanozymes usually exhibit much lower catalytic activities, which limit the sensitivities of nanozyme-based immunoassays. Herein, several metal ions without enzyme-like activities were engineered onto Uio-66-NH₂ nanozyme through postsynthetic modification. The obtained Mⁿ⁺@Uio-66-NH₂ (Mⁿ⁺ = Zn²⁺, Cd²⁺, Co²⁺, Ca²⁺and Ni²⁺) exhibited improved phosphatase-like catalytic activities. In particular, a 12-fold increase in the catalytic efficiency (k_cat/K_m) of Uio-66-NH₂ was observed after the modification with Zn²⁺. Mechanism investigations indicate that both the amino groups and oxygen-containing functional groups in Uio-66-NH₂ are the binding sites of Zn²⁺, and the modified Zn²⁺ ions on Uio-66-NH₂ serve as the additional catalytic sites for improving the catalytic performance. Furthermore, the highly active Zn²⁺@Uio-66-NH₂ was used as a nanozyme label to develop a fluorescence immunoassay method for the detection of cardiac troponin I (cTnI). Compared with pristine Uio-66-NH₂, Zn²⁺@Uio-66-NH₂ can widen the linear range by 1 order of magnitude (from 10 pg/mL-1 μg/mL to 1 pg/mL-1 μg/mL) and also lower the detection limit by 5 times (from 4.7 pg/mL to 0.9 pg/mL).